Probably you have already read the biography of this writer. But I post here an article about his darkest hours which will help you understand better his dark novels. You can also find his biography, just in case you have't found time to read it yet.
It's a bit long but I hope you find time to read it all.
Roald Dahl. Dark hours.
Roald Dahl biography
Quotes
"Children are ... highly critical. And they lose interest so quickly.
You have to keep things ticking along. And if you think a child is getting
bored, you must think up something that jolts it back. Something that tickles.
You have to know what children like."
"As I went on, the stories became less and less realistic and more
fantastic. But becoming a writer was pure fluke. Without being asked to, I
doubt if I'd ever have thought of it."
"A little nonsense now and then is relished by the wisest men."
"The writer for children must be a jokey sort of a fellow. He must
like simple tricks and jokes and riddles and other childish things. He must be
... inventive. He must have a really first-class plot."